The invention relates generally to ground power units employing a charge air cooler. More particularly, the invention relates to a ground power unit with an engine placed upstream from a charge air cooler in a primary air flow path.
Ground power units (GPUs) supply electric power to aircraft when the craft are parked at a terminal, hangar, or other stationary location. Often, GPUs power electrical systems on an aircraft when the aircraft's electric power generation system is disabled. Typically, an aircraft generates electric power by drawing power from its engines. To conserve fuel, pilots turn the engines off when the aircraft is on the ground. However, components in the aircraft often consume electric power while the aircraft is on the ground. For example, a pilot may operate an onboard air conditioning system, communications equipment, lighting, avionics, or other systems when the aircraft's engines are turned off. Thus, when the aircraft is on the ground, it is often connected to a GPU. Typically, a GPU generates electric power that at least partially satisfies the aircraft's needs. The aircraft's electrical systems may continue to operate with power supplied by the GPU, even when the aircraft's engines are turned off. Thus, a GPU may supplement an aircraft's onboard electric power generation system.
Frequently, a GPU includes a stand-alone electric power generation system. For instance, GPUs often include a diesel engine and a generator, wherein the engine drives the generator, creating electrical power. Together, the engine and generator power the operation of electrical systems on the aircraft.
Certain GPUs employ a turbocharger that enhances the emissions performance of the diesel engine. To drive the generator, a diesel engine combusts an air-fuel mixture. A turbocharger may enhance the efficiency of the combustion process. The turbocharger pre-compresses the intake air before the air is introduced into the engine. The engine mixes the pre-compressed intake air with fuel in the cylinders of the engine (in the case of a diesel generator set). Typically, a piston further compresses the air-fuel mixture, which is then combusted. Advantageously, because the intake air is pre-compressed, an engine with a turbocharger may achieve higher pressures in the combustion chamber. Combustion at a higher pressure burns the fuel more completely, reducing emissions.
Typically, as the turbocharger compresses the air, the temperature of the air rises. The compressed air confines this thermal energy of the air circulated through the turbocharger into a smaller volume, raising the air temperature. This thermal energy may increase the heat that the engine must dissipate. Additionally, higher temperature air is less dense than lower temperature air at the same pressure. Lower density air may reduce the effectiveness of pre-compressing the air, as less air enters the combustion chamber. Thus, it may be desirable to cool the pre-compressed air after it leaves the turbocharger.
To enhance the effectiveness of a turbocharger, a GPU often includes a charge air cooler (CAC). The CAC cools the pre-compressed air before it is introduced into the engine. Typically, the pre-compressed air flows through the CAC after (i.e., downstream of) the turbocharger. To cool the pre-compressed air, the CAC typically includes an air-to-air heat exchanger. Often, in addition to the pre-compressed air flowing inside a CAC, a cooling stream of air flows over the CAC. The cooling stream of air removes heat from the pre-compressed air through the air-to-air heat exchanger. The CAC maintains these two air flows separate while facilitating heat exchange between them. Finally, the cooled pre-compressed air flows out of the CAC and into the intake manifold of the engine.
Typically, designers of GPUs desire to reduce the engine's operating temperature without increasing engine noise. Higher operating temperatures can increase wear on the engine components, and engine noise may irritate aircraft passengers and aircraft technicians. Thus, designers may attempt to reduce both engine temperature and noise.
However, these objectives, lower noise emissions and lower operating temperatures, frequently contravene one another. Typically, waste heat energy constitutes much of the energy released by the combustion of fuel in the engine. Designers typically take steps to dissipate this waste heat, such as including a radiator and an air circulation system. However, more powerful air circulation systems often generate more noise. What is more, larger vents associated with greater air circulation often permit more engine noise to escape from the GPU. As a result, designers of GPUs often make tradeoffs between noise emissions and engine temperature.
There is a need, therefore, for an improved design for GPUs, and particularly for turbocharger systems, that permit better cooling of turbocharged air, while maintaining reduced noise levels.